Return to KLUBNL.PL main page

[Top] [All Lists]

Re: LF: Finbar's Compact 500kHz Vertical

To: [email protected]
Subject: Re: LF: Finbar's Compact 500kHz Vertical
From: Rik Strobbe <[email protected]>
Date: Wed, 18 Jun 2008 10:34:59 +0200
In-reply-to: <[email protected]>
References: <[email protected]> <[email protected]> <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Dear HaJo,

I can not find any reason to believe that the same ERP radiated from 2 antennas with the same radiation pattern could produce different field strengths.
But I am far from an antenna specialist or propagation specialist, so maybe others can come up with these reasons.

Another thing:
At the moment we talk about skywave (ionospheric) propagation there might be no such thing as "the best antenna". An antenna that has a high takeoff angle may be superior to an antenna with a low takeoff angle at shorter distances (let's say less than 1000...1500km) while the low angle antenna will do better at long distances.

Regarding DI2AM: provides some figures:
- The TX has 18W input, at 75% efficiency this means about 13 W output.
- The antenna current is +/- 1.7 A, assuming 13W output the total resistance (Rrad + Rloss) is only 5 Ohm.
- The antenna top is 35m asl. Based on the picture I estimate that the antenna bottom is +/- 10m asl, so antenna hight is +/- 25m. The  topload seems about twice as long as the antenna height so let's estimate 50m. According to "mmana-ga"l Rrad is 2.3 Ohm, the antenna capacitance is 840pF (the website notes +/- 800pF, so thta's pretty close) and the gain is 4.64dBi (2.5dBd).
- Putting 1.7A into 2.3 Ohm means 6.65 Watt radiated or +/- 12 Watt ERP.
No wonder they are putting out such strong signal.

73, Rik  ON7YD - OR7T

At 00:51 18/06/2008, you wrote:
Dear Rik,

thank You for your contribution. Especially your last sentence is interesting for me because this may explain the success of MF-stations operating close to the coast.

Another example for exceptionally high antennas on MF for me is DI2AM, located on a museum ship in the harbour of Rostock. I have been informed that the antenna has been put up between two masts of 30 meter in height. Other favourable conditions are the low ground loss onboard of a ship and also the vicinity of water, of course. 

In general I do not have any objections to Finbar's setup; it shows that effective MF antennas can be built up on a rather small area.

But I am not yet convinced that height of MF antennas should not matter, and I will continue to observe the scenery.


73 Ha-Jo, DJ1ZB

  "Rik Strobbe" <[email protected]> schrieb:
Dear HaJo,

But in general I feel that antenna height should not be disregarded. Even with equal ERP sky waves may develop better from higher aerials.


I guess that the only cause that 2 antennas fed with the same ERP produce different signals is a difference in the (vertical) radiation pattern, ie. the takeoff angle.
I simulated a 30m, 10m and 3m vertical (at 502kHz) with mmana_gal and found only very small differences: from 18.2 degrees for the 30m vertical to 18.6 degrees for the 3m vertical. This for a uniform ground with a conductivity of 10mS/m.
Making the ground poor (1mS/m) results in a takeoff angle of about 26 degrees (+/- a few tenths depending on the height).
Increasing the conductivity to 100mS/m reduces the takeoff angle to 12-13 degrees.

73, Rik  ON7YD - OR7T

Disclaimer: for more information.


Disclaimer: for more information.

<Prev in Thread] Current Thread [Next in Thread>